1. Field of the Invention
The invention relates to a continuous granulating process and granulating apparatus for the substantially dust-free preparation of uniform granulated small particles or pellets of quickly hardening reactive plastic mixtures, e.g. which harden quickly after the ingredients are homogenized and ejected from a continuously operating material preparation screw conveyor machine.
2. Brief Description of the Prior Art
Special efforts have been made in the prior art to improve the technology of material preparation because of the problems which arise in the obstruction or clogging of the outlet perforated nozzle plate of the material preparation machine while at the same time meeting the need for a dust-free granulated product. Such dust-free products are required as stable catalysts, as raw materials for smelting processes and for baking. Previously cross-linked macromolecular plastics with or without reinforcing fibers are molded in presses and may also be used as reactive fillers but these harden quickly after homogenization so that screw machinery in which these mixtures are formed becomes obstructed and clogged. It is frequently necessary to include additives to delay hardening. With many mixtures of this quickhardening type, additives are undesirable or prohibited. With other mixtures, such as catalysts almost immediate hardening by an additive permits baking to a dust-free condition. The latter is especially important in preparing noble metal catalysts since recovery of the noble metal from the dust makes catalyst preparation expensive but the additive may not be wanted.
Some proposals for granulation using worm extruders in the past 30 years are described in certain publications as follows:
Schneider and Brooks, Plastics Institute Transactions 26 (1958) Vol. 64 Page 17; PA1 Timm, Stoltzenberg and Fettback, Kautschuk and Gummi, Kunststoffe 18 (1965) Page 206; PA1 Herrmann, Plastics October 1967 Page 1225 FIG. 3 and FIG. 6.
The last named publication shows that the material which leaves the compounding machine is in the form or irregularly formed large pieces. In FIG. 6 of this publication the material is cooled, ground to a fine grain and a dust content of 30% is noted for this conventional process shown in FIGS. 3 and 6. The dust is removed by screening which means a loss in production which may reach 300 kg per hour at a throughput of 1,000 kg per hour. Under favorable conditions, up to 10% of this 300 kg per hour of dust may be blended with the regular granules in the compounding machine for subsequent operations. However, for injection molding dust-free particles are required and similarly dust-free material is required in transfer molding.
It is difficult to justify production of 700 to 800 kg per hour for a total production amount of 1,000 kg per hour. To overcome this problem, a perforated nozzle plate was proposed for the compounding machine and there was added a revolving knife to cut the mixture emerging from the nozzle plate into small pieces. However, the so modified compounding machines fitted with nozzle plate and revolving knife still became obstructed and blocked in a short time. For example, the machine shown in Gresch Swiss Pat. No. 426215 Gresch proposes a deep worm gear with declining core diameter, a length of 1.35 L/P, a high rate of rotation of the worm and a wedge shaped work space between the end of the worm gear. The extruded material in this wedge space is placed under high compression which leads to obstruction at the plate.
In the book "Schneckenmachinen in der Verfahrenstechnik", Springer 1972, Hermann describes the ZSK compounding machine and the ZDS-K compounding machine in detail. At page 128 of this book there is described the possibility of combining these machines with a deep-cut, slow running single cycle ES-A machine having a large diameter and a length of 6 to 8 L/D for use in granulating thermoplastic batches.
The Gresch Swiss patent above was modified according to this Hermann text but this modified procedure was abandoned because, after the first promising experiments, it turned out that continuous operation could not be successfully carried out.